In general, a hydraulic excavator as a representative example of a construction machine comprises an automotive lower traveling structure provided with a crawler belt, an upper revolving structure that is mounted on the lower traveling structure to be able to revolve thereon, and a working mechanism that is mounted to the upper revolving structure to be able to tilt thereto. The hydraulic excavator travels on an irregular ground such as a working site or the like, and performs an excavating operation of earth and sand or the like using the working mechanism.
In this case, the lower traveling structure in the hydraulic excavator is regularly configured of a truck frame that is provided with side frames at both sides of a center frame in the left-right direction to extend in the front-rear direction, an idler wheel that is provided on one end side of the side frame in the front-rear direction, and a drive wheel that is provided on the other end side of the side frame in the front-rear direction. A crawler belt is wound around between the idler wheel and the drive wheel.
The center frame in the truck frame has front legs and rear legs that extend in both sides in the left-right direction, and the left and right side frames are positioned at both sides of the center frame in the left-right direction to be formed extending in the front-rear direction, and are respectively mounted to tip ends of the front leg and the rear leg.
Here, some of large-sized hydraulic excavators among the hydraulic excavators have an outer dimension exceeding a width dimension or a height dimension within a range where the hydraulic excavator can travel on a road defined by regulations. Therefore, in a case of transporting the hydraulic excavator to a working site, the hydraulic excavator is regularly divided into the lower traveling structure, the upper revolving structure and the working mechanism, which are individually transported, and are assembled in the working site. Further, in an extremely large-sized hydraulic excavator used in an open-pit mining or the like, only the lower traveling structure exceeds a dimension within a range where the hydraulic excavator can go through the road. Therefore, the lower traveling structure is divided into three pieces configured of the center frame, and left and right side frame assemblies in which the idler wheel, the drive wheel and the crawler belt are incorporated in each of the left and right side frames. Accordingly, the center frame and the left and right side frame assemblies that are thus configured of the three-divided pieces are individually loaded on different vehicles to be transported to a working site, and after that, are assembled in the working site.
Therefore, the truck frame forming part of the lower traveling structure is configured such that the left and right side frames can be mounted to and removed from the front legs and the rear legs of the center frame. In this case, each of the front leg and the rear leg of the center frame has an end surface at the front end side that is formed in a hollow, square shape, and a front end side of each of the front leg and the rear leg is provided with a mounting flange having a protruding portion protruding closer to an outside than the square end surface. On the other hand, each of the left and right side frames comprises an inner surface plate that is positioned in a side of the center frame and with which the mounting flange makes contact, an outer surface plate that is arranged to face the inner surface plate to be spaced therefrom, and an upper surface plate that connects an upper end of the inner surface plate and an upper end of the outer surface plate.
The inner surface plate of the side frame is provided with plural female screws in positions corresponding to the protruding portion of the mounting flange of each of the front leg and the rear leg, and the protruding portion of the mounting flange is provided with bolt through holes in positions corresponding to the respective female screws. The front end side of each of the front leg and the rear leg is provided with a bracket in which plural bolt through holes are formed, and the upper surface plate of the side frame is provided with plural female screws in positions corresponding to the respective bolt through holes of the bracket. In a case of mounting each side frame to the front leg and the rear leg of the center frame, the bolt is inserted in each bolt through hole in the mounting flange of each of the front leg and the rear leg from the inside in the left-right direction, and a front end side of the bolt is screwed into the female screw of the inner surface plate in the side frame. Further, the bolt is inserted into each bolt through hole of the bracket in each of the front leg and the rear leg from above, and a front end side of the bolt is screwed into the female screw of the upper surface plate of the side frame (Patent Document